1. Field of the Invention
The present invention generally relates to back light units and liquid crystal displays, and more particularly, to a back light unit and a liquid crystal display having a structure by which a foreign body is prevented from invading the space between the back light unit and a liquid crystal panel.
2. Description of the Related Art
There are several types of display apparatuses, such as a liquid crystal display or a cathode ray tube (CRT).
Since the liquid crystal display apparatus has several good points such as low consumption of electric power and a small thickness, marketing of the liquid crystal display apparatus has been expanding. Therefore, the liquid crystal display apparatus needs to have various properties more than the CRT.
The most important property required of the liquid crystal display apparatus is to have high luminance. Particularly, a liquid crystal display is required to have several times the luminance of the related art liquid crystal displays, without increasing manufacturing cost of the liquid crystal display apparatus.
As a means for realizing the high luminance of the liquid crystal display, up to now, using fluorescent tubes, namely lamps, as a light source, the electrical power provided to the individual fluorescent tubes has been increased, or the number of fluorescent tubes has been increased.
However, if high luminance of each of the fluorescent tubes is provided, other members in the liquid crystal display apparatus are thermally influenced by the heat generated by the fluorescent tubes. In addition, if the number of the fluorescent tubes is increased, other members in the liquid crystal display apparatus are thermally influenced and miniaturization of the liquid crystal display apparatus is obstructed. Thus, other means are necessary for realizing high luminance of the liquid crystal display apparatus.
Meanwhile, there are two types of back light units used for the liquid crystal display apparatus, an edge light type and a direct under type.
FIG. 1 is a schematic illustration of a structure of a related art edge light type back light unit.
Referring to FIG. 1, the edge light type back light unit 1a includes plural cold cathode fluorescent tubes 3a. Each of the cold cathode fluorescent tubes 3a faces two side surfaces of the light guide plate 2. Reflectors 4a are provided as the reflectors 4a cover circumferences of the cold cathode fluorescent tubes 3a except an irradiation direction of the cold cathode fluorescent tubes 3a. A reflection sheet 5 is provided at a lower surface (back surface) side of the light guide plate 2. A diffuse reflection pattern 6 is formed on a lower surface of the light guide plate 2. The configuration, measurements, and density of the diffuse reflection pattern 6 are arranged appropriately. A diffusion plate (diffusion sheet) 6a is provided at an upper surface side of the light guide plate 2. A prism sheet 7 is further arranged on an upper surface of the diffusion plate 6a. A liquid crystal panel not shown in FIG. 1 is arranged at an upper position of the prism sheet 7 so that the liquid crystal display apparatus can be obtained.
In the edge light type back light unit 1a, a light radiated from the cold cathode fluorescent tubes 3a irradiates into the light guide plate 2. The light irradiated into the light guide plate 2 is changed to a light having a uniform luminance in a radiation surface of the light guide plate 2 by the diffuse reflection pattern 6. The light radiated from the light guide plate 2 permeates through the diffusion plate 6a and the prism sheet 7 so that a moving direction of the light is controlled. As a result, the liquid crystal panel arranged at the upper position of the prism sheet 7 is irradiated uniformly by the light radiated from the light guide plate 2. Thus, a display having a uniform luminance distribution can be obtained at the liquid crystal panel.
In this case, if consumption of electricity grows in a narrow space where the cold cathode fluorescent tubes 3a are arranged in order to produce the high luminance of the liquid crystal display, other members in the liquid crystal display apparatus are thermally influenced seriously. In addition, it is difficult to increase the number of the cold cathode fluorescent tubes 3a in the narrow space. In a structure in which the number of the cold cathode fluorescent tubes 3a can be increased, an advantage of the edge light type back light unit, namely a capability of making the liquid crystal display apparatus thin, is obstructed.
FIG. 2 is a schematic illustration of a structure of the related art direct under type back light unit.
Referring to FIG. 2, in the direct under type back light unit 1b, the plural cold cathode fluorescent tubes 3b extending to front and back directions in a plane perpendicular to the paper of FIG. 2 are arranged parallel in right and left directions. A lamp house 4b is arranged on a periphery of the line of the plural cold cathode fluorescent tubes 3b as a reflector. A diffusion plate 6b is arranged at an upper position from the cold cathode fluorescent tubes 3b, namely an irradiation side of the cold cathode fluorescent tubes 3b, for example at an upper position of the lamp house 4b. A frame part 8 of the direct under type back light unit 1b has a frame body configuration. The frame part 8 includes a concave part 8a and an opening part 8b. The diffusion plate 6b is grasped between the concave part 8a of the frame 8 and a circumference edge part of the lamp house 4b. An edge part of the frame 8 and a most external circumference edge part of the lamp house 4b are fixed by a screw not shown in FIG. 2. A liquid crystal panel not shown in FIG. 2 is provided on an upper surface of the frame 8 with a spacer 9 having a frame body configuration. The circumference edge part of the liquid crystal panel and the frame part 8 are covered with a frame body not shown in FIG. 2 such as a metal bezel except for a display surface of an upper side of the liquid crystal panel.
In a case of the direct under type back light unit, it is relatively easy and creates little problem to increase the electrical power or increase the number of the cold cathode fluorescent tubes, as compared with the edge light type back light unit. Hence, it is easy for the direct under type back light unit to produce a high luminance of the liquid crystal display.
However, in a case of the direct under type back light unit, there are serious problems with regard to an exchange of the cold cathode fluorescent tube and a dispersion of the luminance of the plural cold cathode fluorescent tubes, as compared with the edge light type back light unit.
That is, in a case of the direct under type back light unit, when the cold cathode fluorescent tube is exchanged, it is necessary to remove the lamp house in which the plural cold cathode fluorescent tubes are arranged. Hence, it is complicated to exchange the cold cathode fluorescent tubes. Furthermore, in a state where the lamp house and the liquid crystal panel are separated, the diffusion plate is removed at a side where the lamp house is removed by removing the lamp house. Therefore, in this case, the liquid crystal panel faces outside air so that an outside foreign body may adhere to the liquid crystal panel. In addition, as shown in FIG. 3, the spacer 9 of the liquid crystal display apparatus assembled completely has a space shown by arrow a in FIG. 3 in order to have measurement tolerances and efficiency in assembling the liquid crystal display apparatus. In this case, even if a space at the back light unit side is made small so that the foreign particle is prevented from invading, the particle may invade from the liquid crystal panel side.
When a foreign body adheres to the liquid crystal panel, a problem such as a defect of displaying regarding the liquid crystal panel occurs. In order to prevent the defect, there is an idea to exchange the cold cathode fluorescent tubes in a clean room. However, it is more complicated to exchange the cold cathode fluorescent tubes in a clean room.
In a case of not only the direct under type back light unit but also the edge light type back light unit, the back light unit is separated based on the need for exchanging the lamps. The same problems in the direct under type back light may occur in the edge light type back light.
Furthermore, the direct under type back light has a structure in which the cold cathode fluorescent tubes are arranged in parallel. Each of the cold cathode fluorescent tubes irradiates only around the circumferences of the each of the cold cathode fluorescent tubes. Hence, it is difficult to maintain a uniform light level, as compared with the edge light type. Accordingly, if the luminance is reduced due to deterioration of one of the cold cathode fluorescent tubes, the reduced luminance causes a non-uniform luminance distribution of the back light unit and eventually of the liquid crystal panel. Therefore, when the consumption of the electricity increases in order to realize high luminance, non-uniform luminance of the liquid crystal panel occurs frequently since the service life of the cold cathode fluorescent tube is shortened.